The name of the local network. The main components and types of computer networks

Hardware, protocols and LAN topology

LAN hardware- this is its server, workstations, network controllers, cable systems, connectors, splitters, repeaters, amplifiers, terminators-stubs.

Server Is a specially dedicated computer on the network with powerful resources, high reliability, connected to a source uninterruptible power supply and equipped with a network OS. It solves the problems of network management and maintenance of its operability. Stores general information, updates its copies for users, conducts backup data, etc.

Work station- this is any computer, other than the server, working on the network.

In practice, it is used two main types of connection computers to LAN - thin Ethernet or twisted pair. Thin Ethernet quite common as the simplest, cheapest, most reliable option. Its elements are network cards with a "bayonet" socket, T-connectors, N-connectors, terminators, thin Ethernet coaxial cable. The cable lengths can be 0.5-185 meters long. The data transfer rate over such a network is 10 (or 100) Mbit / s). Twisted pair there are several types in the implementation. The differences between them are in the level of noise immunity. For twisted pair networks, in addition to network adapters, it is also required additional device- hub (Hub). These networks have the following advantages - high reliability, simple network expansion, the emergence of new standards (including for transmission rates up to 100 Mbit / s), does not require a complete replacement of communication lines, high performance.

Additionally, fiber-optic communication channels and radio channels are also used. The creation and rapid improvement of the Notebook PC led to an equally rapid development wireless technologies connections between PCs, including local networks. Typically, Notebooks have standard ports or slots. This allows you to quickly and easily connect your Notebook to a regular LAN using network cards... The creation of the wireless option LAN requires installation on each workstation or special device server wireless... Networks are implemented either in the radio range, or in laser or infrared communication channels.

The transmission of data over the network is governed by certain rules. The set of rules for interaction between computers on a network is called protocols data transmission, or network protocols. Those. protocol- This is the "language" in which PCs talk to each other on the network. It is installed on them in the form driver software. Protocols define format, method of synchronization, sequence, methods of error handling during data transmission. Transferring data between computers requires many steps.

for instance, to transfer a file from one computer to another, the file must be split into parts, these parts must be grouped in a certain way. Thus, the computer receiving the file should receive Additional information about how the formed groups are related to each other, as well as information about the method of synchronization, information that allows you to correct errors associated with the transfer of data, etc. Given the complexity of communication between computers, this process is usually divided into steps. Each such step is performed in accordance with its own rules, that is, in accordance with its protocol.

Network topology- this is its geometric shape or a diagram of the physical location of the PC in relation to each other and their connection by communication channels. Network topology affects such metrics as reliability, scalability, cost, latency, and throughput.

In practice, three basic topologies are used- bus (EtherNet), ring (Token Ring), star (ArcNet) .

In a network with a star topology, each subscriber that sends and / or receives information is connected by one or two dedicated communication channels to a single central communication center through which all network information passes.

In a ring network, the nodes are connected to signal repeaters that are linked in a unidirectional ring. In a unidirectional ring, a pair of adjacent repeaters are connected by a section of cable - a dedicated communication channel.

In a bus network, all nodes are connected to the same communication channel using transceivers. The channel ends on both sides with passive terminators that absorb the transmitted signals, since by its nature transmission in such a network is broadcast. Bus networks have rather limited scalability due to signal attenuation in the channel. Each insert and each connector slightly changes the characteristics of the physical transmission medium.

The structure of a specific network, its topology and the number of users depend mainly on the structure of the enterprise and the information system tasks.

By the principle of organization, there is two types local area networks: peer-to-peer networks and networks with a dedicated server.

Peer-to-peer LAN- These are networks that do not provide for the allocation of special computers that organize the work. Those. all nodes of such a network perform the same communication functions and are equal. Each user, connecting to the network, allocates some resources to the network (disks, printers) or connects to resources provided to the network by other users. These networks are relatively easy to install. The software for them is primarily Windows 2000 / XP.

In networks with a dedicated server one PC - the server - is a source of software and hardware resources for workstations. A dedicated server LAN can be of the client-server or file-server type. They use specialized network operating systems such as Windows-NT Server or Novell Netware as software. They offer centralized access to network service... Such networks are generally much more efficient and safer (in terms of data protection) than peer-to-peer networks.

Note that Windows 2000 / XP contains components that allow you to work with it both locally and in global networks, both in peer-to-peer networks and with a dedicated server.

Working with Windows on a peer-to-peer LAN is easy. For example, this is "providing a folder or disk to share with other users." To share a drive or folder, you must be logged on as an "expert" user, server operator, or member of the Administrators group. In Windows 2000 Professional, the maximum number of users is 10 (regardless of the number entered in the "no more" box).

Creation of modern Intranets as corporate LANs, assumes data exchange in them based on TCP / IP protocols. The intranet may not have a direct connection to the Internet. In some cases, access to the Internet brings unnecessary problems, especially when you have to work with confidential information... In most cases, however, an Internet connection adds value to the Intranet as it allows access to Internet resources directly from the Intranet Web pages.

MGUS

Institute of Tourism and Hospitality

Course work

in computer science and IT

topic: “ Possible types of local area networks in the office of the company ” .

Teacher:

Gerasimov A.N.

Student: Tsikisova O. A.

group T1-6

Moscow 2000

Local networks 3

Servers 4 Peer-to-peer networks 5Dedicated Server Networks 6Other network software 6 Network controller 8 Print Server 8

Network topology 9

Common bus topology 10 Star topology 12 Ring topology 13 Mixed topology types 15 Ethernet 10Base2 networks 16 Ethernet 10BaseT and 100BaseT networks 18 Switches 19 Other types of networks 20

Local networks

If there are several computers in the same room, building or complex of nearby buildings, the users of which must jointly solve some problems, exchange data or use common data, then it is advisable to combine these computers into a local network.

The local network (sometimes the term "local area network" is used, abbreviated as LAN) is a group of several computers interconnected by cables (sometimes also telephone lines or radio channels) used to transfer information between computers. Connecting computers to a local network requires network hardware and software.

Local networks allow you to provide:

collective data processing by users of computers connected to the network and data exchange between these users;

sharing programs;

sharing printers, modems and other devices.

Therefore, almost all firms with more than one computer integrate them into local networks. Many users laptop computers connect to local network companies either by coming to the office, or by connecting to the company's computer via telephone channels using a modem.

To connect computers to a local network, you need:

insert a network controller into each computer connected to the network (sometimes the terms network adapter or network card are used), which allows the computer to receive information from the local network and transfer data to the network;

connect computers with cables that transfer data between computers, as well as other devices connected to the network (printers, scanners, etc.). In some types of networks, cables connect computers directly (like light bulbs on a Christmas tree garland), in others, cables are connected through special devices - hubs (or hubs), switches, etc.

Comment. In some networks, instead of cables, data is transmitted over radio frequencies (such as radio telephones or cell phones). However, such networks are more expensive and more difficult to operate.

Servers

To ensure the functioning of a local network, a special computer is often allocated - server, or several such computers. The server disks contain shared programs, a database, etc. The rest of the computers on the local network are often called workstations... On those workstations where you only need to process data on the server (for example, enter information into a shared database of orders and sales), often for economy (or for security reasons) they do not install hard drives... In networks of more than 20-25 computers, a server is required - otherwise, as a rule, the network performance will be unsatisfactory. The server is also necessary for joint intensive work with any database.

Sometimes servers are assigned a certain specialization (data storage, programs, provision of modem and facsimile communications, printing, etc.). Servers are usually not used as user workstations. Servers that handle valuable data are often housed in an isolated room that only authorized people have access to (like a bank vault).

Comment. Many servers are much more expensive (10-20 times or more) than conventional computers. It is not surprising - after all, they are not only very powerful computers With large quantity operational and disk memory, but they also provide exceptional reliability, high I / O performance, duplication of devices and stored data, means of monitoring the state of the server, means of ensuring uninterrupted operation when some devices fail, etc.

Local Area Network Software

To ensure the functioning of a local network, you need appropriate software, which we will now talk about

Peer-to-peer networks

OS Windows for Workgroups, Windows 95, Windows NT Workstationhave built-in capabilities for organizing local networks without a dedicated server. Typically, such networks are called peer-to-peer, since in them all computers are equal, each of them performs both the role of a user's workplace and the role of a server to provide access to its data and resources. True, when using Windows for Workgroups or Windows 95 will not be able to protect your data, so such networks can only be used in teams where no one has secrets from each other. You can use other tools for organizing peer-to-peer local networks. For example, Artisoft's OC LANtastic allows you to create a peer-to-peer network that can be run under DOS, Windows, and Windows 95.

Disadvantages of peer-to-peer networks ... Peer-to-peer networking is often not the best solution. After all, the user OS is not well suited to perform the functions of a network server, which it has to perform. And if on some computer the user plays DOOMor draws a picture inAdobe photoshop,and other users work with files on the same computer, they will greatly interfere with each other - the speed of their work will decrease dramatically. And many other features of peer-to-peer networks are very inconvenient - and the lack of information protection, and decentralized data storage, which complicates their backup, and insufficient reliability, and much more. Therefore, usually in local networks, dedicated computers are used that are engaged only in servicing the local network and shared data - servers.

Dedicated Server Networks

In local networks with a dedicated server, the server uses special operating systems that provide reliable and efficient handling many requests from user jobs. On workstations of such a local network, any operating system and the driver must be running to provide access to the local network.

Other networking software

In addition to the network OS, for the effective work of users in the local network, other software is required, which sometimes comes with the network OS, and sometimes it must be purchased separately:

Email ensures the delivery of letters (and often arbitrary files, as well as voice and fax messages) from some users of the local network to others, and sometimes allows you to communicate with remote users by modem or via InterNet;

facilities remote access allow you to connect to a local network using a modem and work on a computer as if it were directly connected to the network (of course, many operations will take longer, since the modem works much slower than the network controller);

group work tools (the most popular of them Lotus Notes) allow you to work together on documents, ensure the consistency of versions of documents for different users, provide tools for organizing the workflow of an enterprise, allow you to organize teleconferences - a written exchange of views on various topics, etc.;

redundancy programs allow you to create backup copies of data stored on servers in the local network and on users' computers, and, if necessary, restore data on their backup;

local network controls allow you to manage the resources of a local network from one workstation, receive information about the status and load of the network, adjust network performance, manage the systems of network users (for example, install software on them), etc.

Equipment for local networks

Network Controller

To connect a computer to a local network, a network controller must be installed in the computer (synonyms - network adapter or network card). The network controller allows the computer to receive information from the local network and transfer data to the network. Network controllers differ from each other in the type of cables to which they can be connected, implemented network protocol(a protocol is an agreement for the exchange of information in a local network), the type of bus to which they are connected ( ISA, PCI, VESA etc.) speed and other qualities.

Print Server

For a printer, the analogue of a network controller is a printer server - a device that allows you to connect a printer to a local network. Typically, a print server is connected to a local network, and is connected to it in the usual way (with a cord or cords such as Centronics) one or more printers. High performance network printers already have a built-in card equivalent to a print server.

Comment. Of course, the printer can also be connected to a local network server (to its parallel port), but this is not so convenient for many reasons. Firstly, the cable for connecting printers can have a limited length - no more than 3, maximum 5 meters. Second, there can be many printers on the network. Third, not all network operating systems support connecting printers to a parallel port.

Network topology

Network topology - This is a logical diagram of the connection by communication channels of computers or network nodes. Most often, the basic topological structures are used, which are of the following nature:

common bus;

annular (annular);

star-shaped.

In order for each of these networks to work, it must have a different access method.

Access method Is a set of rules that determine the use of a data transmission channel connecting network nodes at the physical layer.

The most common access method for LANs listed by topology are:

Ethernet

Token-Ring

Arcnet

Each of these methods is implemented by the corresponding network cards, called the adapter. The network card is physical device, which is installed in every computer connected to the network, and provides transmission and reception of information through communication channels.

Note that the configuration of physical connections is determined by the electrical connections of computers to each other and may differ from the configuration of logical connections between network nodes. Logical links are data transmission routes between network nodes and are formed by appropriate configuration of the communication equipment.

The choice of the topology of electrical connections significantly affects many characteristics of the network. For example, the presence of redundant links increases the reliability of the network and makes it possible to balance the load of individual channels. The ease of attaching new nodes, inherent in some topologies, makes the network easy to expand. Economic considerations often lead to the choice of topologies, which are characterized by the minimum total length of the communication lines.

Common bus topology

Fig. 1 bus topology

A bus topology network uses a single communication channel that connects all the computers on the network.

listen-to-carrier access method frequency and conflict detection. With this access method, the node, before sending data over the communication channel, listens on it and only after making sure that the channel is free, sends a packet. If the channel is busy, the node retries to transmit the packet at a random interval. Data transmitted by one node in the network goes to all nodes, but only the node for which this data is intended recognizes and receives it. Despite the preliminary listening of the channel, conflicts can arise in the network, consisting in the simultaneous transmission of packets by two nodes. Conflicts are associated with the fact that there is a time delay of the signal when it passes through the channel: the signal was sent, but did not reach the node listening on the channel, as a result of which the node considered the channel free and began transmitting.

A typical example of a network with this access method is the network Ethernet. On Ethernetthe data transfer rate for local networks is provided, equal to 10 Mbit / s.

Bus topology ensures efficient use of bandwidth channel, fault tolerance of individual nodes, ease of reconfiguration and network expansion.

A shared bus is a very common (and until recently the most common) topology for local area networks. The transmitted information can be spread in both directions. The use of a common bus reduces the cost of wiring, unifies the connection of various modules, and provides the possibility of almost instant broadcast access to all stations on the network. Thus, the main advantages of such a scheme are the low cost and simplicity of cabling in rooms. The most serious drawback of the common bus is its low reliability: any defect in the cable or any of the numerous connectors completely paralyzes the entire network. Unfortunately, a defect in the coaxial connector is not uncommon. Another disadvantage of the common bus is its low performance, since with this method of connection, only one computer at a time can transmit data to the network. Therefore, the bandwidth of the communication channel is always divided here between all the nodes of the network.

Star topology

Fig. 2 Star topology

A star topology network has an active center (AC) - a computer (or other network device) that unites all computers in the network. The active center fully controls the computers connected to it through a hub, which performs the functions of distributing and amplifying signals. The function of the hub is to direct the information transmitted by the computer to one or all of the other computers on the network. The operability of the network completely depends on the reliability of the active center.

An example of an access method with an AC is Arcnet... This accessor also uses a token to pass data. The token is passed from node to node (as if in a ring), bypassing the nodes in ascending order of their addresses. As in a ring topology, each node regenerates a token. This access method provides a data transfer rate of 2 Mbps.

The main advantage of this topology over the common bus is that it is significantly more reliable. Any troubles with the cable concern only the computer to which this cable is connected, and only a malfunction of the hub can damage the entire network. In addition, the hub can act as an intelligent filter of information coming from nodes to the network and, if necessary, block transmissions prohibited by the administrator.

The disadvantages of a star topology include the higher cost of network equipment due to the need to purchase a hub. In addition, the scope for expanding the number of hosts is limited by the number of ports on the hub. Sometimes it makes sense to build a network using several hubs, hierarchically interconnected by star links.

Ring topology

Fig. 3 Ring topology

The ring topology network uses a closed ring of transmitters and transmitters connected by coaxial or optical cable as communication channels.

In a ring-shaped network, data is transferred from one computer to another, usually in one direction. If the computer recognizes the data as "own", then it copies them to itself into the internal buffer. In a network with a ring topology, special measures must be taken so that in the event of a failure or disconnection of a station, the communication channel between the other stations is not interrupted. The ring is a very convenient configuration for organizing feedback- the data, having made a full turn, is returned to the source node. Therefore, this node can control the process of delivering data to the addressee. This property of the ring is often used to test network connectivity and find a node that is not working correctly. For this purpose, special test messages are sent to the network.

The most common access method in networks of this topology is Token-Ring – token passing accessor .

Marker Is a packet with a special bit sequence. It is sequentially transmitted around the ring from node to node in one direction. Each node relays the transmitted token. A node can transmit its data if it received an empty token. The token with the packet is sent until a node is found to which the packet is intended. At this node, data is received, but the token is not released, but is passed along the ring further. Only after returning to the sender, who can verify that the data he sent was safely received, the token is released. An empty token is passed to the next node, which, if it has data ready for transmission, fills it and passes it around the ring. In networks Token-Ringa data transfer rate equal to 4 Mbit / s is provided.

The relaying of data by the nodes leads to a decrease in the reliability of the network, since a fault in one of the network nodes breaks the entire network.

Mixed topology types

While small networks tend to have a typical star, ring, or bus topology, large networks tend to have arbitrary connections between computers. In such networks, individual arbitrarily connected fragments (subnets) can be distinguished that have a typical topology, therefore they are called networks withmixed topology .

As LANs become more and more widespread, there are problems associated with the exchange of information between networks. So, within the framework of the university, local networks can be used in several classrooms, and these can be networks different types... To provide communication between these networks, a means of internetworking is used, calledbridges and routers ... Computers with 2 or more network adapters can be used as a bridge and a router. Each of the adapters provides communication with one of the associated networks. A bridge or router receives packets sent by a computer on one network to a computer on another network, forwards them and sends them over the specified address... Bridges are usually used to link networks with the same communication systems, for example, to link 2 networks Ethernet or 2 Arcnet networks.Routers connect networks with different communication systems, as they have the means of converting packets from one format to another. There are router bridges that combine the functionality of both. To ensure the connection of the aunt with various computer systems gateways are intended. For example, a local network can be connected to a mainframe via a gateway.

Local area network with multiple file servers

In the event that the local network has a long distance or a large number of subscribers (users), it is advisable to split such a network into smaller networks, each of which must contain its own file server. This leads to the fact that the performance of the network increases, its reliability improves, and if damage occurs in one of the networks, the rest of the network does not lose its operability. A computer acting as a bridge or router is installed one at a time network adapter... Typically, a bridge is used to connect multiple networks with the same communication systems. Packets arriving on the bridge are redirected and sent to another network at the specified address. Routers, if necessary, convert packets from one format to another.

Ethernet 10Base2 networks

The simplest and cheap way building a local network, suitable for connecting 10-20 computers into a network is their connection using a thin cable. Ethernet (Ethernet 10Base2). To do this, a special thin coaxial cable, as thick as a pencil, is laid along all computers connected to the network. This cable is then cut at each computer and a T-shaped connector is inserted into each cut. The third end of the connector (the leg of the “T”) is inserted into the connector on the network controller on the back of the computer. As a result, computers appear to be hung on a network cable, like light bulbs in a garland on a Christmas tree. At the ends of this "garland" special plugs-terminators are inserted. the length of this garland can be up to 200 m - hence the number 2 in the name 10 Base-2. Nominal network bandwidth is 10 Mbps (hence the number 10 in the name). Real bandwidth is about 4-5 Mbps.

Disadvantages of Ethernet 10Base2 networks.

Method of connecting computers using Ethernet 10Base2.

Not always the best. Firstly, it is not very reliable - if any contact in the “garland” is broken, the network seems to break into two pieces or stops working altogether (just like when one light bulb burns out on a Christmas tree garland, the whole garland goes out). Secondly, reworking such a network when adding new computers to it is very difficult. Thirdly, this method has many limitations: maximum length cable, the total number of computers connected to the network, etc. And fourthly, the limitation is 10 Mbps. refers to the total bandwidth of the network, so the network can become congested when sending large amounts of data. Therefore, at present, most networks are created on the basis of special devices - hubs and switches.

Comment

The connector for the thin Ethernet cable is called Bnc, so usually the notation Bnc in the characteristics of a device means that this device can be connected to a local Ethernet 10Base2 network.

The word "coaxial" means that the cable consists of 2 wires, one of which is located around the other, that is, is the braid of the other. There is of course insulation between these wires and around the outer wire. A coaxial cable is used, for example, when connecting TVs to an antenna.

The term Ethernet refers to a type of local area network in which all devices connected to the network hear all signals transmitted over the network and can start transmitting data to the network at any time. Moreover, each transmitted data packet in the Ethernet network contains the number of the device to which it is addressed, so if the device hears someone else's data packet, it ignores it. For each data packet, the receiving device sends back an acknowledgment. And if two devices start simultaneously transmitting data to the network, then they both fall silent and resume transmission after a random period of time. This ensures the arrangement of collisions when transferring data. As long as the network load is low, the network works quite efficiently. But under heavy load, the throughput of the Ethernet network can be noticeably reduced due to frequent collisions when trying to transfer data to the network.

If it is required to create a network with a cable length of more than 200 m, repeaters are installed between the cable segments, which retransmit and amplify the signals coming through the network. There are other options (hub, bridge, etc.).

Ethernet networks 10BaseT and 100BaseT

A more flexible and convenient way to build local networks is to connect computers to a special device called a hub or hub. The hub has several (4, 6, 8, 12, 16, 24, etc.) ports - sockets for connecting network cables through which it connects to computers (more precisely, their network controllers) or other network devices (for example, other hubs). Usually, twisted-pair cables are used to connect a hub and computers - they contain 4 (sometimes 2) pairs of wires, enclosed in a common sheath, with each pair of wires twisted into a cord. These cables are very durable and easy to handle. At the ends of the cables, connectors are installed, very similar to the wire connectors used by imported phones to connect to the telephone socket, only slightly larger in size. For each length of wire, the connector at one end is inserted into the connector on the network controller, and the connector at the other end is inserted into the connector (port) on the hub. Thus, the hub looks like a spider, from which stretches a "cobweb" - wires connecting it to the computers on the network.

When networking up to 30-40 computers, one hub may be sufficient. But often more than one hub is used on a network. For example, each division of an enterprise can have its own hub, and these hubs, in turn, will connect to the main hub of the enterprise.

Switches

The concentrator transmits (relays) incoming messages (data packets) in all directions, except for the one they came from. And the network bandwidth is limited, and under heavy load it decreases due to frequent conflicts when simultaneously trying to transfer data to the network. Instead of a hub, you can use a switch (or switching hub) to increase network bandwidth. It performs the functions of a hub, but incoming messages are transmitted only in the direction in which the recipient of the message is. Thus, the switch, as it were, splits the network into several segments, not allowing messages that are not related to it to pass into each segment. This significantly reduces the load on the network (network traffic). Switches are significantly more expensive than hubs, so there are often no connections to the switch. separate computers, and enterprise division hubs and a server with enterprise shared databases.

Other types of networks

Besides Ethernet networks 10Base-2, 10Base-T and 100Base-T based on electrical cables, other networks are used that differ both in protocol ( Token Ring, FDDIand others) and by data transmission medium. For example, to ensure data transmission over long distances (up to several kilometers) and for faster data transmission, communication lines are used on a fiber-optic cable.

List of used literature:

V. G. Olifer, N. A. Olifer "Computer networks" .- SPb: Publishing house "Peter", 2000.

V. E. Figurnov "IBM PCfor the user ".- M .: INFRA-M, 1997.

A. V. Gavrilov "Local computer networks" .- M .: Publishing house "Mir", 1990.

The term "local network" means a communication system that unites several computers within one room, building or several closely located buildings of one enterprise. Cables, telephone lines, or radio channels can be used to connect computers. Thanks to this combination, users: get access to information on all computers connected to the LAN (programs, folders, files); can share modems, printers, scanners [...]

get access to information on all computers connected to the LAN (programs, folders, files);
can share modems, printers, scanners and other peripheral devices.

The main types of local networks

LANs are classified by different criteria... Consider what local networks are, depending on the presence of a control computer:

Peer-to-peer - they are relevant for combining a small number of computers (up to 10) and in cases where increased information protection is not required. At the same time, users have equal rights of access to information and independently decide which resources of their computer to open public access to. Such networks are extremely easy to install and operate, but it is problematic to provide reliable data protection in them.
Hierarchical, server-based - such LANs provide high performance and reliability of information storage with a large number of users. They have a central computer - a server (one or more), designed for:

centralized LAN control, information exchange and resource allocation;
connection of all peripheral devices;
storage special programs, applications and the main body of LAN information;
development of routes for transmission of messages within the network.

Consider what types of local networks are there by the type of communication:

Wired - such LANs use twisted pair, fiber optic or coaxial cable as communication channels. Twisted pair consists of 2 twisted conductors. It has a low bandwidth (up to 1 Mbps), but it is resistant to interference. Twisted pair of classes 5, 5e, 6, 6e, 7 has a higher bandwidth (from 100 Mbit / s). Coaxial cable contains a central conductor and an insulating coating, on which another conductor (shield) is applied on top. It can be foil or copper wire braid. Fiber optic cable provides speeds in excess of 10 Gbps. It contains a thin glass cylinder on which a cladding with a different refractive index is applied.
Wireless - they work with data transmission in the infrared range or over radio channels. Such networks transmit information quickly, but are vulnerable to interference from other sources of the same frequency and unauthorized external access.

What are the different types of LAN topologies?

Classification of local area networks by topology (structure) is carried out according to the type of connection of computers.

The network configuration can be:

Bus - when a cable connects computers in series with each other. Connection and exchange of information is carried out through a single communication channel - the bus. This topology is simple and economical, providing fast information transfer. But if the cable is damaged, problems arise with the entire network, and it is not easy to identify the location of the malfunction.
Star-shaped - where each machine is connected with a separate cable to a server located in the center of the network, which centrally manages the entire network. Such a structure is more resistant to malfunctions and unauthorized access, does not cause difficulties in locating the problem, and ensures fast data transfer from workstations to the server. It is widely used when creating a local area network. The disadvantages of this structure are low speed data transfer between individual workstations and the dependence of the entire network on the characteristics of the server.
In the form of a ring - when the machines of the network are connected in series with each other and with the server, or jointly perform the functions of a server. Such schemes are distinguished by simplicity of management and quick access to data, but if a separate machine fails or a failure in the channel between 2 nodes, problems arise in the operation of the entire network. To ensure that the communication channel between other computers is not interrupted, special cross-over connections must be used in such networks.
Cellular - a structure in which communication lines are established with all nearby machines. In such systems, only workstations that intensively exchange information are directly connected. Machines not connected by direct links exchange data using transit transmissions using intermediate computers. This structure is suitable for connecting a large number of machines and is often used when organizing wide area or large local area networks. It is maximally resistant to overloads and failures.
Tree-like - is a combination of several topologies (bus-star, star-ring). The base of the LAN tree is at the point where data communication lines are collected. Such networks are in demand when it is impossible to organize basic topologies.
Mixed (hybrid) - when creating large LANs, arbitrary connections between workstations are often used, but subnets with a typical structure can be distinguished.

Now you know what types of local networks exist, how they differ from each other, what are the advantages and disadvantages of each particular type. To decide on the choice of a suitable LAN for you, consult with our specialists. You can also instruct us

in computer science and IT

topic: “ Possible types of local area networks in the office of the company ” .

Teacher:

Gerasimov A.N.

Student: Tsikisova O. A.

group T1-6

Moscow 2000

Local networks ..................................................................................................................................................................... 3

Servers ................................................. .................................................. .................................................. ............................ 4

Local Area Network Software ..................................................................................................... 5

Peer-to-peer networks ................................................ .................................................. .................................................. ........ 5

Dedicated Server Networks .............................................. .................................................. ..................................... 6

Other network software .............................................. .................................................. ................. 6

Equipment for local networks ............................................................................................................................. 8

Network Controller................................................ .................................................. .................................................. ....... eight

Print Server ............................................... .................................................. .................................................. ..................... eight

Network topology ...................................................................................................................................................................... 9

Common bus topology .......................................................................................................................................... 10

Star topology ....................................................................................................................................................... 12

Ring topology ..................................................................................................................................................... 13

Mixed topology types ..................................................................................................................................... 15

Local area network with multiple file servers ..................................................................................... 16

Ethernet 10Base2 networks ......................................................................................................................................... 16

Ethernet 10BaseT and 100BaseT networks ................................................................................................................ 18

Switches ................................................................................................................................................................... 19

Other types of networks .......................................................................................................................................................... 20

List of used literature: ........................................................................................................................ 21

The local network(sometimes the term "local area network" is used, abbreviated as LAN) is a group of several computers interconnected by cables (sometimes also telephone lines or radio channels) used to transfer information between computers. Connecting computers to a local network requires network hardware and software.

Local networks allow you to provide:

· Collective data processing by users of computers connected to the network and data exchange between these users;

· Sharing of programs;

· Sharing of printers, modems and other devices.

Therefore, almost all firms with more than one computer integrate them into local networks. Many laptop users connect to the local network of the company either by coming to the office, or by connecting to the company computer via telephone channels using a modem.

To connect computers to a local network, you need:

Insert a network controller into each computer connected to the network (sometimes the terms network adapter or network card are used), which allows the computer to receive information from the local network and transfer data to the network;

· Connect computers with cables, which transfer data between computers, as well as other devices connected to the network (printers, scanners, etc.). In some types of networks, cables connect computers directly (like light bulbs on a Christmas tree garland), in others, cables are connected through special devices - hubs (or hubs), switches, etc.

Comment. In some networks, instead of cables, data is transmitted over radio frequencies (such as in cordless phones or cell phones). However, such networks are more expensive and more difficult to operate.

Servers

To ensure the functioning of a local network, a special computer is often allocated - server, or several such computers. The server disks contain shared programs, a database, etc. The rest of the computers on the local network are often called workstations... Workstations that only need to process data on the server (for example, enter information into a shared order and sales database) often do not install hard drives to save money (or for security reasons). In networks of more than 20-25 computers, a server is required - otherwise, as a rule, the network performance will be unsatisfactory. The server is also necessary for joint intensive work with any database.

Comment. Many servers are much more expensive (10-20 times or more) than conventional computers. It is not surprising - after all, they are not only very powerful computers with a large amount of RAM and disk memory, but in addition they provide exceptional reliability, high I / O performance, duplication of devices and stored data, means of monitoring the state of the server, means of ensuring uninterrupted operation during failure of some devices, etc.

To ensure the functioning of a local network, you need appropriate software, which we will now talk about

Peer-to-peer networks

Operating Windows systems for Workgroups, Windows 95, Windows NT Workstation have built-in capabilities for organizing local networks without a dedicated server. Typically, such networks are called peer-to-peer, since in them all computers are equal, each of them performs both the role of a user's workplace and the role of a server to provide access to its data and resources. True, with using Windows for Workgroups or Windows 95, data protection cannot be ensured, so such networks can only be used in teams where no one has secrets from each other. You can use other tools for organizing peer-to-peer local networks. For example, Artisoft's OC LANtastic allows you to create a peer-to-peer network that can be run under DOS, Windows, and Windows 95.

Disadvantages of peer-to-peer networks... Peer-to-peer networking is often not the best solution. After all, the user OS is not well suited to perform the functions of a network server, which it has to perform. And if on some computer a user plays DOOM or draws a picture in Adobe Photoshop, and other users work with files on the same computer, then they will greatly interfere with each other - the speed of their work will decrease dramatically. And many other features of peer-to-peer networks are very inconvenient - and the lack of information protection, and decentralized data storage, which complicates their backup, and insufficient reliability, and much more. Therefore, usually in local networks, dedicated computers are used that are engaged only in servicing the local network and shared data - servers.

Dedicated Server Networks

In local networks with a dedicated server, special operating systems are used on the server to ensure reliable and efficient processing of many requests from user workstations. On workstations of such a local network, any operating system can be used and a driver must be running to provide access to the local network.

Other networking software

- Email provides delivery of letters (and often arbitrary files, as well as voice and fax messages) from one local network user to another, and sometimes allows you to communicate with remote users by modem or via InterNet;

- remote access tools allow you to connect to a local network using a modem and work on a computer as if it were directly connected to the network (of course, many operations will take longer, since the modem works much slower than the network controller);

- group work tools(the most popular of them is Lotus Notes) allow you to work together on documents, ensure the consistency of versions of documents for different users, provide tools for organizing the workflow of an enterprise, allow you to organize teleconferences - a written exchange of views on various topics, etc .;

- redundancy programs allow you to create backup copies of data stored on local network servers and users' computers, and, if necessary, restore data from their backup;

- local network controls allow you to manage the resources of a local network from one workstation, receive information about the status and load of the network, adjust network performance, manage the systems of network users (for example, install software on them), etc.

Network Controller

To connect a computer to a local network, a network controller must be installed in the computer (synonyms - network adapter or network card). The network controller allows the computer to receive information from the local network and transfer data to the network. Network controllers differ from each other in the type of cables to which they can be connected, implemented by the network protocol (a protocol is an agreement for the exchange of information in a local network), the type of bus to which they are connected (ISA, PCI, VESA, etc.), speed, and other qualities.

Print Server

For a printer, the analogue of a network controller is a printer server - a device that allows you to connect a printer to a local network. Typically, a print server is connected to a local network, and one or more printers are connected to it in the usual way (with a cord or Centronics-type cords). High performance network printers already have a built-in card equivalent to a print server.

Network topology- This is a logical diagram of the connection by communication channels of computers or network nodes. Most often, the basic topological structures are used, which are of the following nature:

1. common bus;

2. annular (annular);

3. star-shaped.

In order for each of these networks to work, it must have a different access method.

Access method Is a set of rules that determine the use of a data transmission channel connecting network nodes at the physical layer.

The most common access method for LANs listed by topology are:

Each of these methods is implemented by the corresponding network cards, called the adapter. A network card is a physical device that is installed in every computer connected to a network and provides transmission and reception of information over communication channels.

Common bus topology

Fig. 1 bus topology

A bus topology network uses a single communication channel that connects all the computers on the network.

The most common access method in networks of this topology is listen-to-carrier access method frequency and conflict detection. With this access method, the node, before sending data over the communication channel, listens on it and only after making sure that the channel is free, sends a packet. If the channel is busy, the node retries to transmit the packet at a random interval. Data transmitted by one node in the network goes to all nodes, but only the node for which this data is intended recognizes and receives it. Despite the preliminary listening of the channel, conflicts can arise in the network, consisting in the simultaneous transmission of packets by two nodes. Conflicts are associated with the fact that there is a time delay of the signal when it passes through the channel: the signal was sent, but did not reach the node listening on the channel, as a result of which the node considered the channel free and began transmitting.

A typical example of a network with this access method is Ethernet. Ethernet provides a data transfer rate for local networks equal to 10 Mbit / s.

The bus topology provides efficient use of channel bandwidth, fault tolerance of individual nodes, ease of reconfiguration and network expansion.

Star topology

Fig. 2 Star topology

Arcnet can be cited as an example of an AC access method. This accessor also uses a token to pass data. The token is passed from node to node (as if in a ring), bypassing the nodes in ascending order of their addresses. As in a ring topology, each node regenerates a token. This access method provides a data transfer rate of 2 Mbps.

Ring topology

Fig. 3 Ring topology

The ring topology network uses a closed ring of transmitters and transmitters connected by coaxial or optical cable as communication channels.

In a ring-shaped network, data is transferred from one computer to another, usually in one direction. If the computer recognizes the data as "own", then it copies them to itself into the internal buffer. In a network with a ring topology, special measures must be taken so that in the event of a failure or disconnection of a station, the communication channel between the other stations is not interrupted. The ring is a very convenient configuration for organizing feedback - the data, having made a full turn, is returned to the source node. Therefore, this node can control the process of delivering data to the addressee. This property of the ring is often used to test network connectivity and find a node that is not working correctly. For this purpose, special test messages are sent to the network.

The most common access method in networks of this topology is Token-Ring - token passing accessor .

Marker Is a packet with a special bit sequence. It is sequentially transmitted around the ring from node to node in one direction. Each node relays the transmitted token. A node can transmit its data if it received an empty token. The token with the packet is sent until a node is found to which the packet is intended. At this node, data is received, but the token is not released, but is passed along the ring further. Only after returning to the sender, who can verify that the data he sent was safely received, the token is released. An empty token is passed to the next node, which, if it has data ready for transmission, fills it and passes it around the ring. Token-Ring networks provide data transfer rates equal to 4 Mbps.

The relaying of data by the nodes leads to a decrease in the reliability of the network, since a fault in one of the network nodes breaks the entire network.

Mixed topology types

As LANs become more and more widespread, there are problems associated with the exchange of information between networks. So, within the university, in several classrooms, local networks can be used, and these can be networks of different types. To provide communication between these networks, a means of internetworking is used, called bridges and routers... Computers with 2 or more network adapters can be used as a bridge and a router. Each of the adapters provides communication with one of the associated networks. A bridge or router receives packets sent by a computer on one network to a computer on another network, forwards them and sends them to the specified address. Bridges are typically used to link networks with the same communication systems, for example to link 2 Ethernet networks or 2 Arcnet networks. Routers connect networks with different communication systems, as they have the means of converting packets from one format to another. There are router bridges that combine the functionality of both. Gateways are designed to provide aunt with communication with various computer systems. For example, a local network can be connected to a mainframe via a gateway.

In the event that the local network has a large length or a large number of subscribers (users), it is advisable to split such a network into smaller networks, each of which must contain its own file server. This leads to the fact that the performance of the network increases, its reliability improves, and if damage occurs in one of the networks, the rest of the network does not lose its operability. A computer acting as a bridge or router has one network adapter installed. Typically, a bridge is used to connect multiple networks with the same communication systems. Packets arriving on the bridge are redirected and sent to another network at the specified address. Routers, if necessary, convert packets from one format to another.

Ethernet 10Base2 networks

The easiest and cheapest way to build a local network, suitable for connecting 10-20 computers into a network, is to connect them using a thin cable. Ethernet (Ethernet 10Base2). To do this, a special thin coaxial cable, as thick as a pencil, is laid along all computers connected to the network. This cable is then cut at each computer and a T-shaped connector is inserted into each cut. The third end of the connector (the leg of the “T”) is inserted into the connector on the network controller on the back of the computer. As a result, computers appear to be hung on a network cable, like light bulbs in a garland on a Christmas tree. At the ends of this "garland" special plugs-terminators are inserted. the length of this garland can be up to 200 m - hence the number 2 in the name 10Base-2. The nominal network bandwidth is 10 Mbps (hence the number 10 in the name). Real bandwidth is about 4-5 Mbps.

Disadvantages of Ethernet 10Base2 networks.

Method of connecting computers using Ethernet 10Base2.

Not always the best. Firstly, it is not very reliable - if any contact in the “garland” is broken, the network seems to break into two pieces or stops working altogether (just like when one light bulb burns out on a Christmas tree garland, the whole garland goes out). Secondly, reworking such a network when adding new computers to it is very difficult. Thirdly, this method has many restrictions: the maximum cable length, the total number of computers connected to the network, etc. And fourthly, the limitation is 10 Mbps. refers to the total bandwidth of the network, so the network can become congested when sending large amounts of data. Therefore, at present, most networks are created on the basis of special devices - hubs and switches.

Comment

1. The connector for a thin Ethernet cable is called BNC, so usually the designation BNC in the characteristics of a device means that this device can connect to a 10Base2 Ethernet LAN.

2. The word "coaxial" means that the cable consists of 2 wires, one of which is located around the other, that is, is the braid of the other. There is of course insulation between these wires and around the outer wire. A coaxial cable is used, for example, when connecting TVs to an antenna.

3. The term Ethernet denotes a type of local area network in which all devices connected to the network hear all signals transmitted over the network and can start transmitting data to the network at any time. Moreover, each transmitted data packet in the Ethernet network contains the number of the device to which it is addressed, so if the device hears someone else's data packet, it ignores it. For each data packet, the receiving device sends back an acknowledgment. And if two devices start simultaneously transmitting data to the network, then they both fall silent and resume transmission after a random period of time. This ensures the arrangement of collisions when transferring data. As long as the network load is low, the network works quite efficiently. But under heavy load, the throughput of the Ethernet network can be noticeably reduced due to frequent collisions when trying to transfer data to the network.

4. If you need to create a network with a cable length of more than 200 m, repeaters are installed between the cable segments, which retransmit and amplify the signals coming through the network. There are other options (hub, bridge, etc.).

Ethernet networks 10BaseT and 100 BaseT

A more flexible and convenient way to build local networks is to connect computers to a special device called a hub or hub. The hub has several (4, 6, 8, 12, 16, 24, etc.) ports - sockets for connecting network cables, through which it connects to computers (more precisely, their network controllers) or other network devices (for example, other hubs). Usually, twisted-pair cables are used to connect a hub and computers - they contain 4 (sometimes 2) pairs of wires, enclosed in a common sheath, with each pair of wires twisted into a cord. These cables are very durable and easy to handle. At the ends of the cables, connectors are installed, very similar to the wire connectors used by imported phones to connect to the telephone socket, only slightly larger in size. For each length of wire, the connector at one end is inserted into the connector on the network controller, and the connector at the other end is inserted into the connector (port) on the hub. Thus, the hub looks like a spider, from which stretches a "cobweb" - wires connecting it to the computers on the network.

Switches

The concentrator transmits (relays) incoming messages (data packets) in all directions, except for the one they came from. And the network bandwidth is limited, and under heavy load it decreases due to frequent conflicts when simultaneously trying to transfer data to the network. Instead of a hub, you can use a switch (or switching hub) to increase network bandwidth. It performs the functions of a hub, but incoming messages are transmitted only in the direction in which the recipient of the message is. Thus, the switch, as it were, splits the network into several segments, not allowing messages that are not related to it to pass into each segment. This significantly reduces the load on the network (network traffic). Switches are significantly more expensive than hubs, so often not individual computers are connected to the switch, but hubs of divisions of the enterprise and a server with shared databases of the enterprise.

Other types of networks

In addition to Ethernet 10Base-2, 10Base-T and 100Base-T networks based on electrical cables, other networks are used that differ both in the protocol (Token Ring, FDDI, etc.) and in the data transmission medium. For example, to ensure data transmission over long distances (up to several kilometers) and for faster data transmission, communication lines are used on a fiber-optic cable.

1. V. G. Olifer, N. A. Olifer "Computer networks" .- SPb: Publishing house "Peter", 2000.

2. V. E. Figurnov "IBM PC for the user" .- M .: INFRA-M, 1997.

3. A. V. Gavrilov "Local computer networks" .- M .: Publishing house "Mir", 1990.

A local computer network is also called a local computer network, as it serves to combine network devices into a small group. The interconnection of network devices can be carried out not only with the help of cables, but also with the help of wireless technologies.

Purpose of LAN

Consolidation of network devices is its main purpose. Using a LAN, users can exchange data, connect to shared printers and print documents, store data on shared server or on your computer, with the ability to access them by other LAN users.

LAN capacity

Typically, a local area network extends to an office, home, premises or building. A LAN can even connect several buildings, however, if you use a twisted pair cable to connect buildings, the distance (cable length) should not exceed 100 meters. Otherwise, data transmission delays may occur. Twisted pair rarely hesitate to connect two or more buildings into one LAN. Most often, an optical cable and associated equipment are used to accomplish this task.

LAN types

Peer-to-peer LAN

A peer-to-peer local area network is used to connect small amount computers (up to 10 pieces). In a peer-to-peer local area network, each user of his computer makes a decision about access to data for other users of the network. This LAN is also called peer-to-peer.

Server-based local area network

It is a more widespread type of LAN, more efficient and reliable. The server can serve as regular computer, and special, whose features and software are designed specifically for these purposes. The server can perform a lot of functions: store the data of LAN users, assign rights and restrict access to users, determine optimal routes when transmitting messages, and much more.

LAN topology / structure

The topology of a local area network determines the structure of how computers will connect to each other.

1. Bus is a serial connection of computers to a network using a common cable.

2. Star is a parallel connection of computers. Each computer is connected with a cable to one device - a hub or hub.

3. Ring- computers are connected by a cable in an inseparable ring. Failure of any computer or cable break - will lead to inoperability of the LAN.